The present invention relates generally to packaging of microelectronic devices, and more specifically to packaging of electro-microfluidic devices.
Electro-microfluidic devices are electro-mechanical devices fabricated generally in silicon that control or utilize the flow of a fluid (e.g. liquid or gas). These devices may utilize Micro-Electromechanical-Systems (MEMS) elements, e.g. chemical sensors, biosensors, micro-valves, micro-pumps, micro-heaters, micro-pressure transducers, micro-flow sensors, micro-electrophoresis columns for DNA analysis, micro-heat exchangers, micro-chem-lab-on-a-chip, etc. Electro-microfluidic devices typically have very small fluid access ports, e.g. 100 microns; have a small overall footprint (e.g. 3 mmxc3x976 mm); and are commonly made in silicon using processes developed by the MEMS and semiconductor IC industry. A common electro-microfluidic application is ink jet printer heads, which combine electric and fluidic functions on a low-cost, integrated platform.
An integrated microfluidic system incorporates electrical signals on-chip. Such electro-microfluidic devices require fluidic and electrical connection to larger packages. Therefore electrical and fluidic packaging of electro-microfluidic devices is key to the development of integrated microfluidic systems. Packaging is more challenging for surface micromachined devices than for larger bulk micromachined devices. However, because surface micromachining allows incorporation of electrical traces during microfluidic channel fabrication, a monolithic device results.
Despite the commercial success of low-cost ink jet printers, there are no commercially available standardized packages for housing electro-microfluidic devices. Each device has its own customized package, and what works for one device may not be appropriate for another device. Standardized packages do exist for microelectronic integrated circuits, e.g. Dual Inline Package (DIP), and their use has resulted in dramatically reduced costs of design and fabrication. A need exists, therefore, for a low-cost, standardized package suitable for housing an electro-microfluidic device. Common electrical fixtures, such as printed circuit boards, should accept a standardized electro-microfluidic package.
Microfluidic devices have potential uses in biomedical, chemical analysis, power, and drop ejection applications. Typically the use of microfluidics in these applications requires the integration of other technologies with microfluidics. For instance: optical means may be used to sense genetic content, electronics may be used for chemical sensing, electro-magnetics may be required for electrical power generation, or electrical power may be required for thermal drop ejection. The already difficult task of packaging the microfluidic device is compounded by the packaging required for electrical, optical, magnetic or mechanical interconnection. In addition, the full potential of microfluidics can not be realized until many microfluidic devices can be effectively integrated into microfluidic systems. This integration requires effective microfluidic interconnections as well as electrical, and/or optical and other types of interconnections. Of special importance for the application of microfluidics is the integration of electronics with microfluidics. This integration will allow the use of already well developed and extremely useful electronics technology with the newly emerging microfluidics technology.
Surface micromachining of microfluidic devices allows the integrated microfabrication of monolithic chips that contain both electrical and microfluidic devices. Integrated fabrication of surface micromachined Micro-Electro-Mechanical Systems (Integrated MEMS or IMEMS) with electronics has been used to fabricate air-bag accelerometer systems. However, integration of microfluidics and electronics on a single chip has lagged behind IMEMS development at least partly because of the difficulty in packaging microfluidic devices in a leak tight, efficient, inexpensive and reliable manner.
Several different techniques have been used to package microfluidic devices. These techniques do not typically address the problem of making electrical connection as well as fluid connection to the microfluidic devices. The simplest way to make fluid connections is to epoxy or otherwise adhere glass or capillary tubes over holes in the on-chip microfluidic channels. This method is very difficult to implement consistently at the very small scales involved without plugging the holes with adhesive. If one is making many connections, the tediousness and the sensitivity of this method to the amount of shaking in one""s hand make this a very unattractive packaging option. Essentially one is performing very small scale, very meticulous, hand assembly work.
More efficient microfluidic connection techniques have been proposed. Tight fitting fluidic couplers can be used for standard capillary tubes. These couplers are created using Deep Reactive Ion Etching (DRIE) to fabricate cylindrical or annular access holes in a mounting wafer that is fusion bonded to the silicon module containing the microfluidic channels. Capillary tubing fits tightly into these access holes. After fitting the capillary tubes into the couplers, epoxy is applied to the outside of the tubing to seal the connection between the tubes and the couplers. In the most developed version of this technique a plastic fluid coupler fits into the access holes for better alignment and sealing.
A snap-together method can be used to connect microfluidic channels at the wafer scale. Finger micro-joints act as springs that hold the channels together after snapping the wafers into place. The connection is a reversible one. Microfluidic circuitboards have been described. In this package several different microfluidic devices are mounted on a circuitboard that contains embedded flow channels connecting the devices in a microfluidic circuit. Finally, a microfluidic manifold that is created in acrylic and contains channels can feed different microfluidic devices. The different layers of the microfluidic manifold are bonded together using thermal diffusion bonding under 45 psi of pressure.
All of these packaging techniques are typically used with bulk micromachined devices. For surface micromachined microfluidic devices the microfluidic device channels scales are even smaller. For instance, a typical bulk micromachined channel would have a channel depth of 50 to 100 microns (0.002 to 0.004 inches). Whereas a typical surface micromachined channel depth would be 1 to 5 microns (0.00008 to 0.0002 inches). The added challenges of connecting to these smaller microchannels, the limitations of current packaging technology, and the necessity of making electrical as well as fluidic connections to make integrated microfluidic microsystems have led us to develop the following packaging scheme.
Problems with packaging of electro-microfluidic devices can include leaks, plugging of microchannels, corrosion, and contamination of the process fluids by the materials of construction. As the size of electro-microfluidic device continues to shrink, the challenge is making reliable fluidic connections between micro-holes (e.g. 100 microns). A related challenge is reliably transitioning to meso-size holes (e.g. 500 microns), and finally up to miniature-size holes (e.g. {fraction (1/16)} inch OD tubing). No practical solution is commercially available that solves the problem of transitioning fluidic connections from the microscale to the miniature-scale.
Making a reliable fluidic connection between two channels having microscale dimensions (e.g. 100 micron ID) is a critical problem. Conventional O-ring seals are not commercially available in these microsizes, and would be extremely difficult to handle at this scale, and in large numbers. Liquid adhesives, such as conductive epoxies, are commonly used for attaching IC dies to polymeric or ceramic substrates (e.g. die attach). However, the liquid adhesive can flow and plug microfluidic holes or channels during bonding. Likewise, solder sealring joints can suffer from microhole plugging during reflow. What is needed is an adhesive system for making microfluidic connections that can be scaled down to microsized holes (e.g. less than 100 microns) without causing plugging of microholes.
Voldman describes a scheme for making a fluidic connection to a microfluidic chip. See Voldman, Gray, and Schmidt, xe2x80x9cAn Integrated Liquid Mixer/Valvexe2x80x9d, Journal of Microelectromechanical Systems, Vol. 9, No. 3, September 2000, pp. 295-302. As shown in FIG. 1., a threaded screw with a hole drilled through the middle is butted up against a miniature O-ring seal (1 mm OD) that presses against the microfluidic chip, creating an internal compression seal. Small-diameter TYGON tubing (0.5 mm OD) is glued to the hollow screw to provide the fluid. While useful for one-of-a-kind prototype laboratory testing, this scheme is not well suited for miniaturization and mass production as a standardized package. Considering that an electro-microfluidic package may have an array of 20-40 microfluidic access ports on one side, handling this many individual micro O-rings becomes very difficult. Other one-of-a-kind laboratory schemes use glass microcapillary tubes bonded perpendicular to the plane of the electromicrofluidic chip.
Cotofana describes a low-cost transfer mould packaging concept for sensors using a open-window scheme. See Cotofana, et al., xe2x80x9cLow-Cost Plastic Sensor Packaging Using the Open-Window Package Conceptxe2x80x9d, Sensors and Acutators A 67 (1998) pp. 185-190. As shown in FIG. 2, an open-window has been created in the top of a standard transfer-molded plastic encapsulated package housing an electro-microfluidic device wirebonded to an electrical lead frame. A customized lid or cap, having inlet and outlet flow channel access, is glued across the open-window, thereby sealing the open-cavity and providing fluidic access to the upper surface of the sensor chip. Despite using a standardized plastic package, this scheme does not solve the problem of efficiently transitioning fluidic connections from multiple, microsized ports (e.g. 100 microns) located on the chip to the larger diameter connections located on an external fixture. Cotofana""s scheme also constrains the fluid to flow only across the surface of the electro-microfluidic chip, rather than providing individual flow connections to internal channels disposed inside of the chip.
In U.S. Pat. No. 6,136,212, Mastrangelo, et al. describes an electro-microfluidic device (i.e. chip) wirebonded to a standard IC package (DIP or PGA), which includes fluidic interconnects located on the opposite side of the chip that are coupled to fluid access holes ultrasonically drilled through the ceramic package. No details are provided as to how the microfluidic connection is made between the chip and the package, nor between the package and the external fixture. Also, this scheme does not solve the problem of efficiently transitioning fluidic connections from multiple, microsized ports (e.g. 100 microns) located on the device to the larger diameter connections located on an external fixture.
Schuenemann describes a top-bottom ball grid array modular package design that combines electrical and fluidic connections in an integrated package. See M. Schuenemann, et al., xe2x80x9cA Highly Flexible Design and Production Framework for Modularized Microelectromechanical Systemsxe2x80x9d, Sensors and Actuators 73 (1999) pp. 153-168. However, Schuenemann does not disclose how to make the microfluidic connection between the chip and the package. Also, Schuenemann does not discuss the problem of efficiently transitioning fluidic connections from multiple, microsized ports (e.g. 100 microns) located on the device to the larger diameter connections located on an external fixture or supply manifold.
The need remains, therefore, for a standardized electro-microfluidic package that can be plugged into (or surface mounted onto) a fluidic printed wiring board. Fluidic printed wiring boards are standard electrical printed circuit boards that also have fluidic channels embedded inside the board. These channels carry fluid from standard connectors (e.g. {fraction (1/16)} inch OD) located, for example, on the edge of the board to standardized packages or chips directly mounted on the surface of the board. Consequently, a standardized electro-microfluidic package is needed that utilizes standard electronic connections (DIP, PGA, etc.) combined with standardized, highly-reliable fluidic connections, in a small as footprint as possible, suitable for joining to a fluidic printed wiring board, for example. Against this background, the present invention was developed.